The present invention relates to a seal assembly having an elastomeric seal bonded to a static end and a movable end. More particularly, the invention relates to an apparatus and process whereby an elastomeric seal is initially stretched in a direction parallel to its axis and parallel to its comating surface to reduce its cross-section parallel to the comating sealing surface. The stretched seal is installed adjacent the comating surface and then relaxed, permitting the seal to return to an unstretched position and to assume a preloaded position against the comating sealing surface.
Elastomeric seals are in very common use in a wide variety of applications as a means for closing off a passageway (gap) between two parts. The parts are usually metallic and will allow fluids to pass through the gap where the two pieces are joined. To prevent the escape or loss of fluid at these gaps, flexible elastomeric seals are used to close the gap between the two parts. To achieve this function, one side of the elastomeric seal is placed in a cavity or groove in a first part and the exposed side of the seal is comated with the surface of a second part. The prevention of fluid passage through a gap between such parts generally relies upon the maintenance of an initial interference fit with attendant interface biasing forces between the sealing element and the two parts.
This interference fit is termed xe2x80x98presqueezexe2x80x99. Additional interface biasing forces are produced as a consequence of seal distortion and loading by retained pressure. This behavior is well known and is described in the design guides provided by O-ring manufacturers in their technical literature and is generally pertinent to all cross-sectional types of seal, not just circular cross-sections.
Attendant with this interference fit is the risk of physically damaging the seal during its installation by abrasion or cutting from contact with the comating surface during installation. This damage can result from either cutting on edges or from twisting or tearing from excessive frictional drag when presqueeze is excessive. For this reason, the amount of interference fit often must be limited in order to ease or even enable assembly. Seals using plastics with spring expanders instead of elastomers are particularly sensitive to this type of assembly damage.
As fluid pressure is applied to the elastomeric seal, the seal will deform and shift in the direction of the fluid. With time under high pressure loads and/or as the pressure increases, the seal will continue to distort or xe2x80x9ccreep.xe2x80x9d This behavior is further enhanced if the elastomeric seal shrinks in volume or is weakened by its interaction with the retained fluids. Thus, elastomeric seals may lose a substantial portion of their interference fit over time due to creep (xe2x80x98compression setxe2x80x99) or shrinkage volume changes. These problems are significantly amplified as the size of the gap to be sealed is increased.
A frequent solution often used for large gap situations has been to selectively compress the seal by mechanical means after installation in order to achieve adequate presqueeze. This active, rather than passive, approach is commonly used for the basically rectangular cross-section seals used with both tubular and split pipeline repair clamps, as for example in Reneau, U.S. Pat. No. 4,728,125. The seals for these clamps may be either circumferential or linear face seals, and the circumferential seals may also be integral circles or split into semicircles.
Compression applied to the seal by tightening the comating surface against one lateral side of the seal causes the seal to extend the seal radially thereby mating and sealing with the comating member. For example, for typical tubular pipeline repair clamps, jack bolts acting parallel to the tubular axis to provide axial compression on the circumferential seals cause the seals to fully engage with the comating member. This approach often requires later recompression of the seal to offset the unloading effects of creep or volume reduction. In practice, such recompression is an operational nuisance, very expensive, and often impractical.
Stewart and Stevenson of Houston, Tex. have used a different approach to seal installation. Its seal is initially recessed into a groove and then forced into contact with the comating member of the seal by directly applying hydraulic pressure to the back side of the seal groove. Sealing relies upon permanently trapping the actuating pressure behind the seal in its groove. This system is not configured for large gaps, but it does avoid interference fit-induced damages during assembly. However, reliability of such a seal is questionable because the actuating fluid may leak off, relieve the pressure and cause the seal to fail.
The significant areas of performance difficulty for large gaps and high pressures with the cited types of existing seals frequently lead to leaks or complete seal failures. For critical service conditions, such as deep water subsea pipeline repair clamps or hot-tap pipeline fittings, revisiting the clamp for adjusting the compressional preload on installed seals is prohibitively expensive. Further, providing more compressional preload in such cases is not practical for reasons of installation damage to the seal due to excessive interference and increased tendency of the seal to creep and extrude through the gap with high preloads.
Thus, a need exists for a means to install a preloaded seal that does not rely on applying external mechanical force to compress the seal against its comating surface.
The invention contemplates a simple, inexpensive device for eliminating the problems and disadvantages of the prior approaches discussed above. The present invention relates to a seal assembly having an elastomeric seal bonded to both a static end and a movable end. An apparatus and process whereby an elastomeric seal is initially stretched to reduce its cross-sectional thickness perpendicular to the comating sealing surface is described. The stretched seal is installed into the groove or recess and then relaxed, permitting the seal to return to an unstretched position and to assume a preloaded position against the comating sealing surface.
One aspect of the present invention is an elastomeric seal housed in a mounting groove or recess and suitable for sealing large gaps such that the seal will be non-interfering or have minimal interference with its comating surface during installation positioning. Once installed the seal does not require externally applied mechanical compression in order to effect the seal against its comating surface, thereby providing self preloading sealing prior to retention of pressure by the seal. This self preloaded seal is relatively immune to shrinkage and/or creep due to its ability to self-compensate for variations in rubber volume and gap size. The seal is selectably engagable and disengagable and can be set multiple times without damage to the seal.
Another aspect of the present invention is an elastomeric seal configured so that it will be pressure biased against its comating surface when subjected to pressure from one side.
Yet another aspect of the present invention is an elastomeric seal configured so that it will entrap pressure applied from either side in a manner such that it will be assisted in maintaining its contact with the comating surface.
The foregoing has outlined rather broadly several aspects of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed might be readily utilized as a basis for modifying or redesigning the structures for carrying out the same purposes as the invention. It should be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.